Electrophotographic developers are used to develop a latent image into visible form in image forming techniques including electrophotography, electrostatic recording, electrostatic printing, and the like. In a typical development process, a developer is applied to a photoconductive surface having an electrostatic latent image thereon to form a visible image. The developed image is then transferred to a recording medium such as paper in a transfer process, and fixed thereon by fusing the developer in a subsequent fixing process.
There are various types of such electrophotographic developers, including two-component developers formed of carrier and toner particles and one-component developers primarily formed of toner particles that are either magnetic or non-magnetic.
Conventionally, dry toners produced through pulverization processes are widely used in electrophotographic developers. Pulverized toner manufacture involves blending a toner binder, such as styrene or polyester, with other substances including colorant, and melting and cooling the blended materials. The resultant mixture is then crushed and pulverized to form dry toner particles.
Recent studies have investigated use of polymerization processes in toner manufacture, including suspension polymerization, emulsion aggregation, and solution polymerization, where toner particles are obtained through polymerization in an aqueous medium to which a dispersant is added. For example, in a solution polymerization process, toner materials are dispersed and dissolved in a volatile solvent (e.g., a low-boiling-point organic compound), and the resultant solution is emulsified in the presence of a dispersant to form droplets in an aqueous medium. The solvent contained in the droplets is removed by subsequent volatilization so that the droplets contract in volume to form small solid particles. The solution polymerization process is superior to the other polymerization processes because of its ability to handle a wide range of resins including polyesters, which can be used to produce color toners with enhanced transparency and smoothness of printed images.
However, the polymerization methods mentioned above have several drawbacks due to the use of dispersant in an aqueous phase, which make their application to toner manufacturing less successful. For example, dispersant remaining on toner particles after polymerization may affect charging properties and degrade environmental stability of the toner, and removing such dispersant residues by washing requires large quantities of water.
Alternatively, a known process using spray drying can obtain toner particles without involving dispersion in an aqueous medium. In spray drying production of toner, a liquid prepared by melting or dissolving toner materials is atomized into fine droplets, and toner particles are obtained by drying the liquid droplets. Unfortunately, conventional spray drying processes do not offer toner particles with desired properties, such as smooth surface, small size, narrow particle size distribution, etc., and various methods have been proposed in an attempt to provide high quality spray-dried toner.
For example, one approach uses a piezoelectric pulse source to actuate a nozzle which dispenses droplets of liquid upon application of pulses. Another version of this approach includes dispensing droplets by means of a thermal expansion at a nozzle actuated by a piezoelectric pulse source. Particulate toner is produced by drying the liquid droplets obtained through such processes. Both of these methods use a piezoelectric element dedicated to a single dispensing nozzle, which results in a low granulation rate, and therefore do not offer satisfactory productivity.
Another spray drying method uses a piezoelectric transducer as a vibration actuator. The vibration actuator is annular in shape and surrounds and supports multiple nozzles connected thereto. In use, the vibration actuator regularly contracts and expands to induce vibrations so that the multiple nozzles regularly vibrate to discharge droplets of liquid, and particles are obtained by solidifying the droplets in a subsequent drying process.
Although advantageous in terms of productivity, such a method does not provide satisfactory reliability and validity for toner manufacture applications. One reason is unevenness in vibration amplitude among the multiple nozzles, which results in lack of uniformity and homogeneity of the resulting liquid droplets and toner particles. Further, according to this method, the vibration is conducted only to an area defined by the annular transducer, which makes the nozzles susceptible to blocking when the material liquid contains a high proportion of solid contents and has a relatively high viscosity, e.g., 10 millipascal seconds (mPa·s), limiting its applicability to toner production processes.
Another method using a piezoelectric vibration actuator is also proposed in which the vibration actuator is connected to a perforated member having multiple nozzles, and droplets are dispensed through the nozzles when the perforated member vibrates. Such a method has not been described in detail, and still does not provide an adequate solution to the drawbacks encountered by the above-mentioned techniques.